Why would a glass shower door spontaneously shatter?

I heard what sounded like a small explosion coming from the bathroom upstairs, so I ran to see what was going on…the shower door appeared to have just exploded, for no reason! Our house is less than 2 years old, and I don’t know who the glass manufacturer is. There wasn’t a breeze or an open window to slam it shut (I think it was closed anyway). Just wondering if anyone has ever heard of this happening and what caused it??